This invention relates to a dual-circuit brake system which includes a master cylinder with two master-cylinder pistons the work chambers of which connect with one brake circuit each, which has coaxially to the master cylinder a pressure-control valve for introducing pressure supplied from a power source into the dual-circuit brake system, and which permits the master-cylinder pistons to be actuated by displacing a portion of the pressure-control valve if there is a failure of the power source. Such a dual-circuit brake system is known from the German unexamined published patent application DT-OS No. 2,045,803.
In this known dual-circuit brake system, depression of the brake pedal will cause pressure to be introduced from the pressure-control valve into a booster chamber. This pressure acts on an actuation surface of a master-cylinder piston, thereby displacing the latter and causing it to urge brake fluid out of its work chamber. The brake pressure which builds up will act on a second master-cylinder piston, thereby enabling the latter to similarly urge brake fluid out of its work chamber.
A disadvantage of the brake system of DT-OS No. 2,045,803 is that the volume available in the two brake circuits is limited by the volume of the two master-cylinder piston work chambers. Even if the power source is fully available no pressure increase can occur any longer in the brake circuits once the two master-cylinder pistons have covered their maximum possible travel. Such a case may happen, for instance, if air has entered into the brake circuits. The fact that the brake-fluid volume available in both brake circuits is very limited makes it impossible to install in the brake circuits a simple antiskid arrangement wherein brake fluid is withdrawn to reduce the pressure.
From U.S. Pat. No. 3,639,008, a brake system is known wherein the brake pedal activates both a master cylinder and a pressure-control valve through two separate tappets. Pressure fluid from a power source is supplied to the wheel brake cylinders by the pressure-control valve. Also, there is a connection between the pressure-control valve and the master cylinder. When the brake is applied, the pressure introduced by the pressure-control valve prevents movement of the master-cylinder piston. In this manner, the master-cylinder piston is at all times available for an emergency braking when the power source has failed.
Compared to the system DT-OS No. 2,045,803, the brake system known from U.S. Pat. No. 3,639,008 has the advantage that a sufficient amount of brake fluid is available at the wheel brake cylinders. However, the provision of two separate brake-actuating arrangements renders the system rather costly and complicated. It has also to be considered that two lines must lead to each brake. A defect in the connecting line between the wheel brake cylinders and the power source may make it impossible to operate the brake through the master cylinder. If it is desired to enlarge the system so that also in a dual-circuit brake system both circuits can be activated independently from one another through a master cylinder upon the failure of the power source, the expenditure therewith involved will be multiplied accordingly.